more readme

Author: sentrivana

MIGRATION_GUIDE.md

@@ -19,12 +19,13 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 - Redis integration: In Redis pipeline spans there is no `span["data"]["redis.commands"]` that contains a dict `{"count": 3, "first_ten": ["cmd1", "cmd2", ...]}` but instead `span["data"]["redis.commands.count"]` (containing `3`) and `span["data"]["redis.commands.first_ten"]` (containing `["cmd1", "cmd2", ...]`).
 - clickhouse-driver integration: The query is now available under the `db.query.text` span attribute (only if `send_default_pii` is `True`).
 - `sentry_sdk.init` now returns `None` instead of a context manager.
+- The `sampling_context` argument of `traces_sampler` now additionally contains all span attributes known at span start.
 
 ### Removed
 
 - Spans no longer have a `description`. Use `name` instead.
 - Dropped support for Python 3.6.
-- The `custom_sampling_context` parameter of `start_transaction` has been removed.
+- The `custom_sampling_context` parameter of `start_transaction` has been removed. Use `attributes` instead to set key-value pairs of data that should be accessible in the traces sampler.
 - The PyMongo integration no longer sets tags. The data is still accessible via span attributes.
 - The PyMongo integration doesn't set `operation_ids` anymore. The individual IDs (`operation_id`, `request_id`, `session_id`) are now accessible as separate span attributes.
 - `sentry_sdk.metrics` and associated metrics APIs have been removed as Sentry no longer accepts metrics data in this form. See https://sentry.zendesk.com/hc/en-us/articles/26369339769883-Upcoming-API-Changes-to-Metrics